I think you're given that those are points on a quadratic equation, ie this is not an estimation. just assume the form ax^2 + bx + c = y and make 3 equations using any 3 y/x relationships you know and solve the system of 3 unknowns for a, b, and c.

I ran an rref on a matrix like so and got the right answer:
[3^2, 3, 1, 36.7]
[5^2, 5, 1, 27.5]
[7^2, 7, 1, 20.7]